Treatment of spent caustic waste

ABSTRACT

Systems and methods are provided for the treatment of caustic wastewater. Specifically, systems and methods are provided for combining refinery spent caustic and ethylene spent caustic solutions and treating the combined spent caustic mixture using a wet air oxidation process.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §120 as acontinuation of U.S. patent application Ser. No. 12/677,642, titledTREATMENT OF SPENT CAUSTIC WASTE, filed on Mar. 11, 2010, which isNational Stage Entry under 35 U.S.C. §371 of International ApplicationNo. PCT/US08/10625, filed Sep. 11, 2008, which claims priority under 35U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/971,386,titled REDUCTION OF pH OF SPENT CAUSTIC PRIOR TO WAO FOR INCREASEDPERFORMANCE; Provisional Application Ser. No. 60/971,400, titledCOMBINING SPENT CAUSTIC FROM ETHYLENE PRODUCTION AND PETROLEUM REFININGFOR WET AIR OXIDATION; and U.S. Provisional Application Ser. No.60/971,410, titled pH ADJUSTMENT TECHNIQUES, each of which were filed onSep. 11, 2007, and each of which is herein incorporated by reference inits entirety for all purposes.

BACKGROUND OF INVENTION

1. Field of Invention

The present disclosure is directed toward methods and systems for thetreatment of caustic wastewater and, more specifically, to the treatmentof caustic wastewater by wet air oxidation.

2. Discussion of Related Art

Wet air oxidation is a well-known technology for treating processstreams, and is widely used, for example, to destroy pollutants inwastewater. The method involves aqueous phase oxidation of undesirableconstituents by an oxidizing agent, generally molecular oxygen from anoxygen-containing gas, at elevated temperatures and pressures. Theprocess can convert organic contaminants to carbon dioxide, water, andbiodegradable short chain organic acids, such as acetic acid. Inorganicconstituents including sulfides, mercaptides, and cyanides can also beoxidized. Wet air oxidation may be used in a wide variety ofapplications to treat process streams for subsequent discharge,in-process recycle, or as a pretreatment step to supply a conventionalbiological treatment plant for polishing.

Wet air oxidation units are in many cases operated at specifictemperature and pressure ranges and sometimes with the addition ofspecific catalysts, depending on the nature and composition of theprocess stream to be treated. Operating conditions suitable for thetreatment of some process streams may not be suitable for the treatmentof different process streams.

SUMMARY OF INVENTION

In accordance with an embodiment of the present disclosure there isprovided a method for treating a waste stream. The method comprisesproviding an ethylene production spent caustic comprising at least afirst organic compound, providing a refinery spent caustic comprising atleast a second organic compound, combining at least a portion of theethylene production spent caustic with at least a portion of therefinery spent caustic in a ratio sufficient to produce a combinedstream comprising a chemical oxygen demand (COD) level below that whichresults in the precipitation of carbonate during wet air oxidation ofthe combined stream, and oxidizing the combined stream at an elevatedtemperature and a super-atmospheric pressure sufficient to treat atleast a portion of the second organic compound.

In some aspects of the method, the second organic compound is anaphthenic compound and in some aspects, the second organic compound isa cresylic compound.

Some aspects of the method may further comprise monitoring a pH of theoxidized combined stream with a pH sensor and adding a pH adjuster tothe combined stream responsive to a signal from the pH sensor. Inaccordance with some aspects, the pH adjuster may be carbon dioxide, andin others, the pH adjuster may be a base.

In some aspects, the step of oxidizing the combined stream may compriseoxidizing the combined stream in a wet air oxidation unit comprising ahead space and introducing carbon dioxide into the head space of the wetair oxidation unit.

Some aspects of the method further comprise biologically treating atleast a portion of the oxidized combined stream. Some aspects furthercomprise adjusting the pH of a portion of the oxidized combined streamprior to the biological treatment.

According to some aspects of the method, the step of combining at leasta portion of the ethylene production spent caustic with at least aportion of the refinery spent caustic comprises combining at least aportion of the ethylene production spent caustic with at least a portionof the refinery spent caustic in piping of a system used to treat thecombined stream. According to other aspects, the step of combining atleast a portion of the ethylene production spent caustic with at least aportion of the refinery spent caustic comprises combining at least aportion of the ethylene production spent caustic with at least a portionof the refinery spent caustic in a mixing vessel.

Some aspects of the method further comprise adding at least one of aminewastewater, sour wastewater, wash water, decant slop, and biologicalsludge to the combined spent caustic stream.

According to some aspects of the method, oxidizing the combined streamcomprises oxidizing the combined stream at a temperature in a range ofbetween about 200 degrees Celsius and about 280 degrees Celsius.

In some aspects of the method, the COD level of the combined stream isless than about 125,000 mg/L.

In accordance with a further embodiment of the present disclosure, thereis provided a system for treating a waste stream. The system comprises awet air oxidation unit comprising an inlet and an outlet, a source ofethylene production spent caustic in fluid communication with the inletof the wet air oxidation unit, and a source of refinery spent caustic influid communication with the inlet of the wet air oxidation unit.

According to some aspects, the system further comprises a pH sensorconfigured to detect a pH of an effluent of the wet air oxidation unit,a controller coupled to the pH sensor, and a source of pH adjustercoupled to the controller and in fluid communication with the wet airoxidation unit.

Some aspects of the system further comprise a source of carbon dioxidein fluid communication with the wet air oxidation unit. According tofurther aspects, the system further comprises a source of a base influid communication with the wet air oxidation unit.

In accordance with some aspects, the wet air oxidation unit comprises awet air oxidation unit formed from stainless steel, and according tosome aspects, the wet air oxidation unit comprises a wet air oxidationunit formed from titanium.

Some aspects of the system further comprise a biological waste streamtreatment unit in fluid communication with and downstream of the wet airoxidation unit.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic of a spent caustic waste treatment systemaccording to an embodiment of the present disclosure;

FIG. 2 is a flow chart of a method of treating spent caustic wasteaccording to an embodiment of the present disclosure; and

FIG. 3 is a schematic diagram illustrating various possible sources ofwaste that may be treated in a wet air oxidation unit in accordance withan embodiment of the present disclosure.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing,” “involving,” and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

In industries such as petroleum refining and ethylene production,aqueous caustic washing is sometimes employed to improve the quality ofthe product and/or aid in the refining process. The caustic washings aredone to remove, for example, sulfidic and/or acidic components from therelevant hydrocarbon streams. Aqueous spent caustic solutions from thesetreatments may contain various contaminants such as sulfides,mercaptans, naphthenates, cresylates, and emulsified hydrocarbons. Theaqueous spent caustics may often have high pH levels, for example, pHlevels of about 13 or above. Environmental and safety considerations maymake treatment of the spent caustic desirable before it is discharged tothe environment.

Wet air oxidation (WAO) may be used to treat some types of spent causticwaste streams. WAO may treat the spent caustic by oxidizing the sulfidesand mercaptans to sulfate and breaking down the naphthenic and cresyliccompounds. The treated spent caustic may be further treated in abiologic treatment system. Depending on the type of spent caustic andthe concentrations of the various contaminants contained therein, designand operating parameters such as temperature, pressure, hydraulicretention time, pH adjustment methodologies, and post treatmentmethodologies may be tailored for the treatment of a particular wastestream.

This disclosure will describe a number of waste streams that may betreated alone or in combination in a WAO system. This disclosure willalso describe systems and methods for treating a combination ofdifferent caustic waste steams by WAO. Specifically, this disclosurewill describe systems and methods that have been discovered thatfacilitate the dilution of refinery spent caustic with ethylene spentcaustic to form a combined caustic stream, and treatment of the combinedstream in a common WAO vessel. For example, one aspect of a methoddescribed in this disclosure comprises combining an ethylene productionspent caustic comprising at least a first organic compound, and arefinery spent caustic comprising at least a second organic compound ina ratio sufficient to produce a combined stream comprising a COD levelbelow that which results in the precipitation of salts such as sodiumcarbonate during wet air oxidation of the combined stream. For example,the combined stream may have a COD level less than 125,000 mg/L, and insome aspects of the invention the combined stream may have a COD ofabout 1000,000 mg/L. The combined stream may be oxidized at an elevatedtemperature and a super-atmospheric pressure sufficient to treat atleast a portion of the second organic compound.

In one embodiment, the heat released during oxidation of a combinedstream comprising a COD of about 100,000 mg/L may be sufficient tosustain the reaction but not generate a significant amount of excessheat which would be removed from the wet oxidation unit. It has beenfound that a COD of greater than about 125,000 mg/L of the combinedstream may result in difficulty controlling the heat in the system. Ithas also been found that a COD of greater than about 125,000 mg/L of thecombined stream may result carbonate precipitation at particulartemperatures, pH and the efficiency of the organic oxidation.

As used in this disclosure, the term “refinery spent caustic” refers tospent caustic generated in the operation of equipment and processes suchas those which may be found at a petroleum refinery. The term “ethylenespent caustic” refers to spent caustic generated in the operation ofequipment and processes such as those which may be found at an ethyleneproduction facility, such as caustic used in the scrubbing of ethylene.Refinery spent caustic may have high levels of chemical oxygen demand(COD), in some cases between about 400,000 mg/L and 500,000 mg/L ormore, while ethylene spent caustic may have a much lower level of COD,in some cases 30,000 mg/L or less. Refinery spent caustic may containone or more of naphthenic spent caustics, cresylic spent caustics, andsulfidic spent caustics. Ethylene spent caustics may contain sulfidescarbonates and a small fraction of organic compounds.

Naphthenic spent caustics may be produced from the scrubbing of keroseneand jet fuels and may contain high concentrations of organic compoundsconsisting of naphthenic acids, and also may contain phenol compoundsand reduced sulfur compounds. Naphthenic spent caustics may also containhigh levels of chemical oxygen demand (COD), in some cases greater than100,000 mg/L. Naphthenic spent caustics may also contain thiosulfatesand naphthenic acids, which may be broken down in a wet air oxidationprocess at temperatures above about 220° C. to about 280° C. or higher.The characterization of the contaminants and properties of a number ofdifferent samples of naphthenic spent caustics is illustrated below inTable 1:

TABLE 1 Characterization of Naphthenic Spent Caustics Sample typeNaphthenic Naphthenic Naphthenic Naphthenic Naphthenic Naphthenic SampleDescription Kero Merox Kero Merox LLCN GTU S.C. Ex Electrostatic S.C.Caustic Caustic Settler Coalescer Combined Washer S.C. MCN SampleLocation V809 V1613 V1602 TR 269 V705 V60 Analytical Reported ResultsUnits As Chemical Oxygen mg/L O₂ 72,200 38,400 16,500 49,600 158,000126,000 Demand Dissolved mg/L C 442 2,170 60 391 2750 380 InorganicCarbon Non Purgable mg/L C 23,600 13,900 4,240 6,250 28,500 43,500Organic Carbon Biological mg/L O₂ 53,600 11,700 6,980 17,500 65,600106,000 Oxygen Demand BOD: COD ratio — — 0.74 0.30 0.42 0.35 0.42 0.84Total Sulfur mg/L S 1,050 236 171 12,100 40,600 2,020 Potentiometricmg/L S 342 <30 <30 11,100 45,700 <30 Sulfide Potentiometric mg/L CH₃SH<30 <30 <30 432 1,450 <30 Mercaptan Sulfate mg/L S 83.4 29.6 19.2 30.731.8 131 Sulfite mg/L S 331 6 86 29 29 368 Thiosulfate mg/L S₂O₃ 3,090325 271 650 2,170 5,690 Phenolics mg/L C₆H₆O 20,700 732 1,570 2,61022,800 37,800 Phenol mg/L C₆H₆O <460 <9.2 <9.2 <46 10,100 <460Naphthenic Acids mg/L — 27,800 6,730 8,240 8,760 8,220 67,800 (DieselRange Organics) p Alkalinity mg/L CaCO₃ 255,000 47,000 26,900 62,500208,000 296,000 m Alkalinity mg/L CaCO₃ 262,000 60,800 28,300 84,800299,000 305,000 Specific Gravity — — 1.192 1.070 1.024 1.072 1.218 1.224Total Sodium mg/L Na 117,000 36,900 13,900 41,300 151,000 138,000 PH — —13.1 13.3 13.4 13.2 13.0 12.8

Sulfidic refinery spent caustics may be produced from the scrubbing ofhydrocarbons such as liquefied petroleum gas products and may containhigh concentrations of reduced sulfur compounds, such as sulfides andmercaptans, and may have low organic carbon concentrations. Thecharacterization of the contaminants and properties of a number ofdifferent samples of sulfidic spent caustics is illustrated below inTable 2:

TABLE 2 Characterization of Sulfidic Refinery Spent Caustics Sample typeSulfidic Sulfidic Sulfidic Sulfidic Sulfidic Sulfidic Sample DescriptionVBU GRU 2 Poly 2 Poly & LPG 2 Poly & Caustic Caustic H2S Abs 1 LPGCaustic 1 LPG Washer Sample Location V503F V503E VH16 V54 V-57 C401Analytical Results Units Reported As Chemical Oxygen mg/L O₂ 96,60067,700 46,900 8,120 84,800 5,980 Demand Dissolved Inorganic mg/L C 871,700 107 292 186 290 Carbon Non Purgable Organic mg/L C 2,840 1,540 6351,440 2,150 1,410 Carbon Biological Oxygen mg/L O₂ <20000 10,000 <60002,530 6,510 1,250 Demand BOD: COD ratio — — <0.21 0.15 <0.13 0.31 0.080.21 Total Sulfur mg/L S 24,400 8,540 23,000 950 10,600 1,000Potentiometric Sulfide mg/L S 29,300 8,920 22,900 981 10,600 752Potentiometric mg/L CH₃SH 17,800 20,100 2,790 2,340 61,200 955 MercaptanSulfate mg/L S 456 41.2 28.5 87.3 56.5 53.2 Sulfite mg/L S 13 12 280 6369 23 Thiosulfate mg/L S₂O₃ 16,300 434 13,100 3,250 3,680 867 Phenolicsmg/L C₆H₆O 1,400 263 10 0.76 0.59 <0.500 Phenol mg/L C₆H₆O <460 120<0.46 <0.46 <0.46 <0.46 Naphthenic Acids mg/L — 2,140 737 <250 <250 <250<250 (Diesel Range Oganics) p Alkalinity mg/L CaCO₃ 55,000 240,000191,000 261,000 187,000 265,000 m Alkalinity mg/L CaCO₃ 93,500 265,000227,000 267,000 205,000 271,000 Specific Gravity — — 1.080 1.182 1.1641.182 1.144 1.188 Total Sodium mg/L Na 48,400 114,000 107,000 111,00095,600 122,000 Ph — — 13.1 13.2 13.2 13.2 13.3 13.2

Cresylic spent caustics may be produced from the scrubbing of gasolineand may contain high concentrations of phenol compounds (cresylic acids)and can also contain reduced sulfur compounds.

Ethylene spent caustic may be similar to sulfidic spent caustic, but mayhave relatively lower levels of sulfur compounds and a lower COD.Ethylene spent caustic may come from the caustic scrubbing of crackedgas from an ethylene cracker. This liquor may be produced by a causticscrubbing tower. Ethylene product gas may be contaminated with H₂S(g)and CO₂(g), and those contaminants may be removed by absorption in thecaustic scrubbing tower to produce NaHS(aq) and Na₂CO₃(aq). The sodiumhydroxide may be consumed and the resulting wastewater (ethylene spentcaustic) contaminated with the sulfides and carbonates and a smallfraction of organic compounds. Insoluble polymers resulting from thecondensation of olefins during scrubbing may also be present. Somecomponents which may be found in ethylene spent caustic are shown belowin Table 3:

TABLE 3 Components of a Representative Ethylene Spent CausticRepresentative Range Compound of Amount Present NaHS 05.%-6%   Na₂CO₃1%-5%  NaOH 1%-4%  NaSR  0%-0.2% Soluble Oil 50 ppm-200 ppm Benzene 20ppm-100 ppm

WAO is an aqueous phase oxidation process using molecular oxygencontained in air (or any other oxygen containing gas) as an oxidant. Theprocess may operate at elevated temperatures and super-atmosphericpressures. Some WAO systems may operate at temperatures and pressureswhich may range from about 120° C. (248° F.) to 320° C. (608° F.) and760 kPa (110 psig) to 21,000 kPa (3000 psig), respectively. Some systemsmay operate at temperatures as high as the critical temperature ofwater, 374° C. Other systems may operate at even higher temperatureswherein the fluid being treated in the vessel may exist at least in partas a supercritical fluid. The utilization of higher treatmenttemperatures may reduce the amount of time required for a desired levelof treatment.

In some systems the pressure of the reaction vessel may be controlled toa specific set point, and in others the pressure of the reaction vesselmay attain a certain level as a result of the heating of the fluid beingtreated and the atmosphere within the sealed vessel.

In some WAO systems the wastewater or feed liquor to be treated ispumped up to pressure by a high pressure feed pump. A gas stream, suchas air, containing sufficient oxygen to meet the oxygen demandrequirements of the waste stream may then be injected into thepressurized waste stream, and the air/liquid mixture may be preheated tothe desired reactor inlet temperature. The mixture may then beintroduced into a reactor vessel where the majority of oxidation maytake place. Alternatively, or in addition, oxygen containing gas mayalso be injected directly into the WAO reaction vessel. Some WAO systemsalso include subsystems allowing the pH of the waste stream to betreated to be adjusted. A pH adjuster, such as an acid or a base, may beadded to the stream to be treated before introduction into the WAOreactor vessel, or into the reactor vessel itself.

The WAO reactor may provide sufficient retention time to allow theoxidation to approach a desired reduction in COD. Oxidation reactions,being exothermic, typically produce a temperature rise in the reactor,making the reactor outlet temperature higher than the inlet temperature.This temperature differential may allow for the recovery of heat fromthe hot reactor effluent. The hot reactor effluent may be used, forexample, to preheat the feed to the reactor.

In some cases, there is more thermal energy available than is requiredfor preheating the reactor feed. Even after heating the reactor feed,therefore, the reactor effluent may still require cooling beforedischarge.

After cooling, the pressure of the reactor effluent stream may bereduced and separated into vapor and liquid phases. The liquid phase maybe transferred or discharged to a further treatment system, such as abiological treatment plant for final polishing. The vapor phase may befurther treatment or released to the environment.

Naphthenic spent caustics often have a high COD, and as a result, arenot readily biodegradable. These spent caustics may have an odor due tothe organic and reduced sulfur compounds contained therein and may havea tendency to foam. Some WAO units for treating naphthenic spentcaustics may operate at temperatures and pressures sufficient to treatat least one organic compound present in the naphthenic spent caustic.For example, the naphthenic spent caustic may be treated in a wet airoxidation unit at a temperature ranging between about 220° C. and about370° C. at a sufficient pressure to maintain at least a portion of thecaustic as a liquid. The naphthenic spent caustic may be treated in awet air oxidation unit at a temperature ranging between about 240° C.and about 320° C. at a pressure and a hydraulic retention timesufficient to achieve a desired COD destruction. Other systems mayoperate in a temperature range of between about 240° C. and about 260°C. with a hydraulic retention time of about 60 minutes. Due to thecorrosive nature of naphthenic acids, a high nickel alloy may be usedfor the materials of construction of a WAO system for the treatment ofnaphthenic spent caustic. During wet air oxidation, the naphthenic acidsmay be oxidized to carbon dioxide (which forms carbonate in an alkalinesolution) and short chain organic acids such as acetic and oxalic acid.Phenolic compounds, which may be present in some naphthenic spentcaustic, can be treated in a temperature range of between about 220° C.and about 260° C. and thus are treatable within the temperature range atwhich the naphthenic compounds may be treated. As with the naphtheniccompounds, the phenolic compounds are converted to carbon dioxide andshort chain organic acids. During WAO, reduced sulfur compounds areconverted to oxidized sulfate compounds.

The reactions associated with the oxidation of sulfide species in wetair oxidation are as follows:2NaHS+2O₂→Na₂S₂O₃+H₂O  1.NaHS+3/2O₂→NaHSO₃  2.NaHS+2O₂→NaHSO₄  3.NaSR+O₂→NaHSO₄+RCOOH (Unbalanced)  4.

Naphthenic and Cresylic compounds may be oxidized by the followingreactions:[Naphthenics]+O₂→CO₂+RCOONa (Unbalanced)  5.[Cresylics]+O₂→CO₂+RCOONa (Unbalanced)  6.

where R may be, for example, CH₃.

In general, reactions 1 and 2 can be achieved at low temperatures, forexample, below about 200° C., and may be the primary reactions whichoccur in WAO systems operating at temperatures between about 100° C. toabout 200° C. Reactions 3 and 4 are generally limited to systemsoperating at temperatures between about 200° C. and about 260° C. orsystems operating at temperatures between about 260° C. and about 374°C. where all reduced sulfur is converted to sulfate.

With lower temperature systems operating at 150° C. (302° F.) or less,the effluent will contain thiosulfate, sulfite, and sulfate as theoxidation products. Increasing temperature, retention time, and oxygenpartial pressure will tend to increase the percentage of reduced sulfurconverted to sulfate and correspondingly reduce the amount ofthiosulfate and sulfite produced.

The extent of oxidation of the organic fraction of the spent causticwill typically be very small in lower temperature systems. At highertemperatures, for example 190° C. (374° F.) and above, an organic CODreduction of approximately 50 percent or greater can be achieved.

Sulfidic spent caustics often have a high COD and, accordingly, are notreadily biodegradable. These spent caustics may also have a strong odordue to the reduced sulfur compounds present in the spent caustic. SomeWAO units for treating sulfidic spent caustics may operate attemperatures ranges of between about 180° C. and about 260° C. with ahydraulic retention time sufficient to achieve a desired CODdestruction. Some WAO units for treating sulfidic spent caustics mayoperate at a temperature of about 200° C. with a hydraulic retentiontime of about 60 minutes. At least a portion of the COD may beattributed to reduced sulfur compounds, such as sulfides and mercaptans.During WAO, the reduced sulfur compounds are converted to sulfate andsulfonic acids. In some cases it is desirable to treat sulfidic spentcaustic streams at temperatures of 200° C. or greater. This is due tothe incomplete oxidation of sulfide which forms thiosulfate attemperatures less than 200° C. The formation of thiosulfate can add asignificant COD load to a biological treatment unit that may be utilizedin a post-treatment operation following WAO. By treating the sulfidicspent caustic at 200° C. or greater, the thiosulfate is reduced to nearnon-detectable ranges and H₂S is readily oxidizes to sulfate (SO₄),typically with a COD destruction of greater than 99%.

WAO treatment of ethylene spent caustic may be performed at lowertemperatures (less than about 175° C.), which may not be sufficient fortreating refinery spent caustics, as noted above. Although it has beendiscovered that the treatment of ethylene spent caustic at highertemperatures may be feasible, this has not typically been done due tothe increased power and associated operating cost associated withmaintaining an ethylene spent caustic WAO treatment vessel at elevatedtemperatures. In addition, elevated treatment temperatures may result inhigher pressures within a WAO reaction vessel. Thus, if ethylene spentcaustic were to be treated at a higher temperature, a more pressureresistant vessel might be required than might be required if operatingat lower temperatures. Such a vessel may cost more than a vessel inwhich ethylene spent caustic is conventionally treated.

In some cases, it is desirable to maintain a COD level in a spentcaustic solution to be treated in a WAO system below that which mayresult in the precipitation of salts such as sodium carbonate during wetair oxidation of the combined stream. It may also be desirable tomaintain the COD level below that which may generate excess heat beyondthat which the wet air oxidation unit may handle. In some aspects, itmay be desirable to maintain a COD level in a spent caustic solution tobe treated in a WAO system above a level which will allow the oxidationreactions to produce enough heat to be self-sustaining, but low enoughto reduce the potential for the precipitation of solids, such as a CODlevel of less than about 125,000 mg/L. In one embodiment the COD levelused may range between about 20,000 mg/L and about 100,000 mg/L.

Dilution of a high COD spent caustic, such as a refinery spent caustic,may be performed to achieve a desired COD level in a spent causticstream to be treated by WAO. For example, for the treatment of refineryspent caustic, which may have a COD level of between about 400,000 mg/Land 500,000 mg/L or above, dilution of the caustic influent may bedesirable. Dilution of refinery spent caustic is typically performedwith water to reduce the COD level of the caustic waste to be treated tobelow that which may result in the precipitation of salts such as sodiumcarbonate during wet air oxidation of the combined stream, and such thata degree of heat beyond that which the system may handle is notgenerated, for example, below about 125,000 mg/L. However, there aredrawbacks to this process. The dilution of refinery spent caustic withwater increases the total volume of caustic solution to be treated,thus, in some cases requiring larger and/or a greater number of WAOunits to be provided, which may increase the capital costs associatedwith the WAO system. A greater amount of oxidized spent caustic mayrequire post treatment, which may increase operating costs. Also, thereare costs associated with providing dilution water and reclamation ofthis water.

Treatment of a combined stream of refinery spent caustic and ethylenespent caustic utilizing WAO has provided unexpected results. In someembodiments of the invention of the present disclosure, refinery spentcaustic, ethylene spent caustic, and in some aspects, other wastefluids, may be combined in a ratio to produce a mixture with a COD belowthat which may result in the precipitation of salts such as sodiumcarbonate during wet air oxidation of the combined stream, and such thata degree of heat beyond that which the system may handle is notgenerated, for example, a COD level of less than about 125,000 mg/L. Insome aspects, a target COD of the mixture may be set at between about20,000 mg/L and about 100,000 mg/L. The combined stream may be oxidizedat an elevated temperature and a super-atmospheric pressure sufficientto treat at least a portion of the COD of the combined stream, includingat least a portion of an organic compound provided in the refinery spentcaustic, such as a naphthenic or cresylic compound.

Systems for performing this method may comprise a wet air oxidation unitcomprising an inlet and an outlet, a source of ethylene production spentcaustic in fluid communication with the inlet of the wet air oxidationunit, and a source of refinery spent caustic in fluid communication withthe inlet of the wet air oxidation unit.

The oxidation reactions of the combined refinery spent caustic and theethylene spent caustic surprisingly do not interfere with one another,and the presence of both may even assist the treatment of the two spentcaustics. Conventional practice is to treat these waste streamsseparately due to the differences in treatment conditions required tobreak down the different contaminants of interest in the different wastestreams and the differences in the way that the oxidized spent causticfrom each process are post-treated. It was unexpected that the reactionsresulting in the oxidation of the components of the different wastestreams could proceed and produce an acceptable level of COD reductionwithout interfering with one other.

In many cases, refinery spent caustic has a COD level that is too highto be treated “as is” in the WAO system. In these cases, dilution wateris typically added and the size and/or run rate of the WAO system may beadjusted to account for the dilution water. Ethylene spent caustictypically has a lower COD level than refinery spent caustic, and thusmay be at least partially or completely substituted for the dilutionwater. The dilution water used to dilute the refinery spent caustic maythus be at least partially or completely replaced with a waste streamthat one may also desire to treat. It may be more efficient to dilutewith the ethylene spent caustic and treat both the ethylene spentcaustic and the refinery spent caustic at the same time rather than usewater to dilute the refinery spent caustic and treat it separately fromthe ethylene spent caustic. Capital costs may be reduced because onecommon WAO vessel may be used instead of having separate vessels andattendant pumps, piping, and the like for treating the two differentwaste streams independently. Operating costs may be reduced by reducingor eliminating the need for providing and reclaiming dilution water, andby reducing the total amount of oxidized caustic that may be subject topost treatment.

The presence of ethylene spent caustic in the mixed caustic stream to betreated may also reduce energy consumption, facilitating reduction ofoperating costs with regard to the provision of power. The reaction ofconverting sulfides that may be present in the ethylene spent caustic tosulfate is exothermic. This additional heat can be utilized and reduceany heat input required to heat the refinery spent caustic to anappropriate temperature for treatment by WAO, for example an temperatureof between about 240° C. and about 320° C., that in some embodiments mayfacilitate treatment.

Further, dilution of refinery spent caustic with ethylene spent causticmay allow for superior pH control of the oxidized caustic. Diluting therefinery spent caustic with ethylene spent caustic instead of water mayhelp prevent the pH of the oxidized spent caustic mixture from droppingto a point at which the oxidized solution might damage the WAO reactionvessel. This is because the ethylene spent caustic may counteract theeffect on pH of acids produced during the oxidation of the refineryspent caustic. Naphthenic acids produced by the oxidation of refineryspent caustic may cause the pH of refinery spent caustic diluted withwater to drop more significantly than when the ethylene caustic ispresent in the combined caustic. The presence of ethylene caustic maybuffer the acid production in the naphthenic spent caustic.

The effluent from a WAO unit may be treated such as by pH adjustment tofacilitate further downstream treatment. For example, the WAO effluentmay be transferred to a biological treatment unit as a post-treatmentstep. For this, the effluent may be pH adjusted, for example, to a pHlevel of between about 6.5 and 8, although an even narrower range of pHmay be desired. With oxidized ethylene spent caustics there is littlebuffering capacity, so the oxidized caustic may be adjusted from a highpH to a low pH with only a small amount of acid, resulting in grossadjustment control. pH control of oxidized ethylene spent caustic maytherefore be difficult because the amount of acid added to the oxidizedethylene spent caustic may need to be tightly controlled. With theaddition of refinery spent caustic to the ethylene spent caustic, duringoxidation, the CO₂ produced by the oxidation of the refinery spentcaustic combines with the caustics and forms carbonate. The carbonatehas a significant buffering capacity. The pH of the effluent of acombined mixture of refinery spent caustic and ethylene spent caustictreated by WAO may change more slowly with the addition of acid, makingit easier to adjust the effluent pH into a desired range.

Refinery spent caustic may be produced continuously or in batches. Thus,systems for treating refinery spent caustic and ethylene spent caustictogether in a single WAO vessel may include buffer or storage tanks forstoring either ethylene or refinery spent caustic until a desired amountof each is available. These systems may also include mixing tanks orother forms of mixing systems where the caustic streams are mixed inbulk in sufficient amounts to achieve a desired COD in the combinedstream. The different caustic streams may also be combined in-line atappropriate ratios during the operation of a WAO treatment system.

Systems according to some embodiments of the present disclosure are notlimited to treating only combinations of refinery spent caustic andethylene spent caustic. Other waste streams may also be combined withthe spent caustic streams for treatment in a WAO system. Examples ofthese additional waste streams include amine wastewater, sour wastewater(as used herein, the term “sour” refers to liquid streams and waterstreams that contain a high content of sulfur, hydrogen sulfide, and/orammonia), wash water, decant slop, and biological sludge.

In some WAO systems, the reaction vessels may be constructed comprisingmaterials such as nickel, that are resistant to cracking brought aboutby exposure to highly caustic solutions, but which are subject tocorrosion at low pH levels. In such systems it may be desirable to addadditional caustic, such as NaOH, to the spent caustic waste to betreated in the WAO vessel to counteract any effect of the reduction inpH due to the production of acidic reaction products.

Other WAO systems may include reaction vessels comprising materials suchas stainless steel or titanium, which are more resistant to corrosionunder acidic conditions than nickel, but which are more susceptible toattack by caustic solutions. In these systems, caustic waste streams tobe treated may be pH adjusted, for example to below 12 by the additionof an acid such as sulfuric acid or carbon dioxide.

The wet air oxidation unit may comprise one or more control systems,such as pH, temperature and/or pressure control systems, to facilitatemore efficient operation of a WAO unit for the treatment of spentcaustic. For example, the pH of one or both of the caustic mixture to betreated or the effluent of the WAO system may be monitored by a pHsensor or sensors, and a pH adjustment system activated to adjust the pHof the caustic mixture to be treated in response to a detected pH level.The pH adjustment system may include one or more pH monitors or sensors,a controller coupled to or in electrical communication with the one ormore pH sensors, and one or more sources of pH adjuster. The pH adjustermay comprise acids such as sulfuric acid, carbon dioxide, and/or basessuch as NaOH. The pH adjuster may be added at various locations in theWAO system, such as at an inlet to the WAO vessel, at an outlet of theWAO vessel, in the WAO unit itself, or in any of the various pipesassociated with the system, and in fluid communication with the WAOunit.

An embodiment of a system for treating a combined stream of refineryspent caustic and ethylene spent caustic is illustrated schematically inFIG. 1. The refinery spent caustic enters the system through line 102and enters holding/buffer tank 104. The ethylene spent caustic entersthe system through line 106 and enters holding/buffer tank 108. Holdingtanks 106 and 108 may be omitted in some aspects of the embodimentillustrated in FIG. 1. Either periodically or continuously, refineryspent caustic and ethylene spent caustic may be pumped by pumps (notshown) into mixing tank 110 to form a mixed caustic solution. Mixingtank 110 may also serve as a buffer tank for the mixed caustic solution.The mixed caustic solution then proceeds from mixing tank 110 throughhigh pressure pump 112, through heat exchanger 114, and into WAOreaction vessel 116. In heat exchanger 114, the mixed caustic solutionmay be pre-heated by absorbing heat from oxidized mixed caustic exitingthe WAO reaction vessel 116. Either before or after passing through highpressure pump 112, a pH adjuster, such as an acidic compound (such assulfuric acid and/or carbon dioxide) or a base (such as NaOH) may beintroduced into the mixed caustic from source of pH adjuster 118.Alternately, or in addition to source of pH adjuster 118, a source ofcatalyst (not shown) may also be provided.

The mixed caustic is treated in the WAO reaction vessel 116 at anelevated temperature and pressure to form an oxidized mixed caustic. Anoxygen containing gas such as air may be introduced into WAO reactionvessel 116 from source of oxygen containing gas 120.

A pH adjuster, such as carbon dioxide from pH adjuster source 122 may beintroduced into the WAO reaction vessel 116 along with the oxygencontaining gas from source 120. A pH adjuster, such as carbon dioxide,in solid, liquid, or gaseous form, may also be introduced into theheadspace of WAO reaction vessel 116 from pH adjuster source 124. Theintroduction of carbon dioxide into the head space of the WAO vessel mayallow for a finer degree of pH control of the mixed caustic solutionthan direct addition of acid.

Oxidized mixed caustic may exit WAO reaction vessel 116 through line 126and pass through heat exchanger 114, where it transfers heat to mixedcaustic to be treated in WAO reaction vessel 116. The oxidized mixedcaustic may then proceed to liquid/vapor separation unit 128. At one ormore points along the line or lines carrying the oxidized mixed causticmay be a pH monitor 130 which may detect the pH of the oxidized mixture,and in response to the detected pH, a controller (not shown) maygenerate a signal to control the addition of pH adjuster from any or allof pH adjuster sources 118, 122, and 124.

In liquid/vapor separation unit 128, vapors, such as carbon dioxide, areseparated from the liquid oxidized caustic mixture and released throughvapor line 132 to be recycled, and/or further treated, and/or releasedto the environment. Liquid exiting liquid/vapor separation unit 128 mayprecede to a further treatment system, such as biological treatment unit134, and then be recycled, and/or further treated, and/or released tothe environment. Liquid exiting liquid/vapor separation unit 128 mayalso be pH adjusted by source of pH adjuster 136 in response to a pHreading from pH monitor 130, or from another pH monitor (not shown).

In alternate aspects of the disclosed system, multiple WAO vessels 116may be utilized in series or in parallel, and any of a number ofpost-treatment systems, such as membrane filtration, biologicaltreatment, hydrocyclone separation, etc. may be used.

FIG. 2 is a flow chart illustrating a method according to an embodimentof the present disclosure for treating spent caustic. In step 202refinery spent caustic is provided. In some aspects, the refinery spentcaustic may be produced intermittently or periodically, and may beprovided to a holding or a buffer tank prior to treatment. The refineryspent caustic may in some aspects be a combination of one or more ofnaphthenic spent caustic, cresylic spent caustic, and sulfidic spentcaustic. In step 204, an ethylene spent caustic is provided. Theethylene spent caustic may be provided to a holding or a buffer tankprior to treatment. In some aspects, the refinery spent caustic and theethylene spent caustic may be provided from the same facility. In otheraspects, the refinery spent caustic and the ethylene spent caustic maybe generated simultaneously at one facility or at different facilitiesand/or at different times.

In step 206, the refinery spent caustic and the ethylene spent causticare combined. In some aspects, additional waste fluids, such as aminewastewater, sour wastewater, wash water, decant slop, and biologicalsludge may also be combined with the spent caustics at this step. Thiscombination may take place in a mixing vessel, or in some aspects may beperformed by injecting the spent caustics and/or other waste fluidsdirectly into a line leading to a treatment system. The refinery spentcaustic, ethylene spent caustic, and in some aspects, other wastefluids, may be combined in a ratio to produce a mixture with a COD ofless than that which may result in the precipitation of carbonatesand/or less than that which would make temperature control of the WAOunit difficult during WAO treatment, such as less than about 125,000mg/L. In some aspects, a target COD of the mixture may be set at betweenabout 20,000 mg/L and about 100,000 mg/L.

In optional step 208, the pH of the mixture may be adjusted. The pH maybe adjusted so that the mixture does not damage the WAO vessel by beingtoo caustic or by becoming too acidic as a result of oxidationreactions. In some aspects, especially those where a treatment vesselconstructed of a material such as nickel which is resistant to attack bycaustic solutions is utilized, the pH of the mixture may be adjustedupward by the addition of a base such as sodium hydroxide. This may helpprevent the oxidized mixed caustic from becoming acidic and attackingthe WAO vessel. The dilution of the refinery spent caustic with ethylenespent caustic may provide a similar benefit in that the presence of theethylene spent caustic may buffer the pH of the mixed caustic solution.

In other aspects, the pH of the mixture may be adjusted downward by theaddition of an acid such as sulfuric acid, or by the addition of carbondioxide. In some aspects the pH of the mixture may be adjusted to belowabout 12. In these aspects, a WAO treatment vessel constructed ofstainless steel or titanium might be utilized. In some aspects, reducingthe pH of the mixed caustic may increase the efficiency of the WAOtreatment (see Example 2 below).

In step 210, the mixture is transferred to a treatment vessel where wetair oxidation of the mixture may take place. The mixture may bepre-heated by passing through a heat exchanger before entering thetreatment vessel. In some aspects, wet air oxidation of the mixture maybe performed at a temperature and pressure sufficient to break downcomponents such as naphthenic acids in the caustic mixture. Thetemperature at which the wet air oxidation unit may be operated may bein a range as high as from about 200° C. to about 320° C. In otheraspects, the temperature at which the wet air oxidation unit may beoperated may be in a range as of from about 200° C. to about 280° C. Inone embodiment, the temperature of the wet air oxidation unit may rangefrom about 240° C. to about 320° C., or in a range of from about 240° C.to about 260° C. Components such as phenols and mercaptans may also beoxidized under these conditions. Higher or lower temperatures may beutilized depending on the composition of the caustic mixture to betreated and/or the degree of COD destruction desired.

Sufficient oxygen-containing gas is typically supplied to the system tomaintain residual oxygen in the wet oxidation system off gas, and thesuperatmospheric gas pressure is typically sufficient to maintain waterin the liquid phase at the selected oxidation temperature. For example,the minimum system pressure at about 240° C. is about 33 atmospheres,and in some embodiments may be about 80 atmospheres. At 220° C. apressure of between about 40 and about 60 atmospheres may be used. Theminimum pressure at about 280° C. is about 64 atmospheres, and theminimum pressure at about 373° C. is about 215 atmospheres. In oneembodiment, the aqueous mixture is oxidized at a pressure of about 30atmospheres to about 275 atmospheres. The wet oxidation process may beoperated at an elevated temperature below about 374° C., the criticaltemperature of water. In some embodiments, the wet oxidation process maybe operated at a supercritical elevated temperature. The retention timefor the aqueous mixture within the reaction chamber should be generallysufficient to achieve the desired degree of oxidation. In someembodiments, the retention time may be least about 15 minutes and up toabout six hours. In some embodiments, the retention time may range fromabout 60 min to about 90 min. In other embodiments, the retention timemay be about one hour.

An oxygen containing gas, such as air, oxygen enriched air (aircontaining greater than 21% oxygen), or pure oxygen may be provided tothe wet air oxidation unit during the treatment of the caustic mixture,although any oxidizing agent may be used (for example, ozone, peroxide,permanganate, or combinations thereof). A pH adjuster, such as carbondioxide, may also be provided to the wet air oxidation unit during thetreatment of the caustic mixture. The pH adjuster may be provided alongwith the oxygen containing gas, or may be provided into the wet airoxidation unit at a different location, such as the head space of theWAO vessel. The caustic mixture may in some aspects have a hydraulicretention time of between about 60 and about 90 minutes in the WAOvessel, although shorter or longer hydraulic retention times may beutilized depending on the composition of the caustic mixture to betreated and/or the degree of COD destruction desired.

After oxidation in the WAO vessel, the oxidized spent caustic mixturemay leave the vessel, by pumping or otherwise, and be post-treated (step212). Post-treatment may involve pH adjustment, such as pH adjustment ofthe oxidized mixture toward a neutral pH. Post-treatment may alsoinvolve liquid/gas separation and further treatment of the separatedgas, liquid, or both. The liquid may be further treated in a biologicaltreatment unit to break down or neutralize remaining impurities and/ormay be treated in a filtration system, such as a membrane filtrationsystem. The gas and/or the post treated liquid may then be recycled orreleased to the environment. For example, if carbon dioxide is recoveredfrom the oxidized mixture, it may be re-used as a pH adjustment agent inthe treatment system.

FIG. 3 is a schematic diagram illustrating various potential sources andpotential flow rates of sludges and spent caustics and variouspre-treatment steps that may be applied prior to WAO of a mixture ofthese sludges and spent caustics. From this figure it can be seen thatadditional waste may be treated in a WAO unit in addition to ethylenespent caustic and refinery spent caustic, 302 and 304 respectively,includes sludge from a process water decanter 306, biological sludge308, oily sludge from a sediment tank in an oily wastewater line 310,sludge from the decantation of slops 312, and oily sludge from thebottom of an API decanter (a separator designed to American PetroleumInstitute standards) in an oily treatment train 314.

FIG. 3 also illustrates other processes that may be performed on thevarious waste streams prior to WAO treatment. For example, oily sludgemay be passed through a thickener to recover some water that may besubject to further post treatment. Waste oils may be removed from thesludge streams by a process of pH adjustment and oil separation by meansof a hydrocyclone or other wise. Also, a pre-softener, such as Na₂CO₃may be added to the waste stream prior to WAO.

EXAMPLES Example 1 Comparison of the WAO Treatment of Refinery SpentCaustic Diluted with Water Versus Refinery Spent Caustic Diluted withEthylene Spent Caustic

A mixture of approximately 74% ethylene spent caustic and 16% refineryspent caustic was treated in a WAO unit as shown in FIG. 1 at 240° C.with a hydraulic retention time of 90 minutes. The COD destruction leveland the pH of the effluent of the reaction vessel were measured. Thesequantities were compared to those achieved by treating a naphthenicspent caustic under the same conditions. As can be seen in Table 4below, the COD destruction of the mixed caustics was approximately 84%while the COD destruction of the naphthenic spent caustic diluted withwater was approximately 79%. Also, the data shows that the ethylenespent caustic buffered the pH of the refinery spent caustic during theoxidation process; the oxidized refinery spent caustic diluted withwater displayed a pH of 8.2 while the oxidized refinery spent causticdiluted with ethylene spent caustic displayed a pH of 8.4. This datashows that a mixture of ethylene spent caustic and refinery spentcaustic may successfully be treated in a WAO unit under conditions thatmay be used to treat a naphthenic spent caustic diluted with water. Thismay lead to an increased efficiency of operation in that a singleprocess may be utilized to treat the two different streams of spentcaustic, allowing for the elimination of separate systems for thetreatment of the different spent caustics.

TABLE 4 Refinery Spent Refinery Spent Caustic Diluted Reported CausticDiluted With Ethylene Units As With Water Spent Caustic ChargeConditions Reaction ° C. — 240 240 Temperature Residence Time minutes —90 90 Spent Caustic Composition Naphthenic Spent % — 100 14.8 CausticCresylic Spent % — — 2.2 Caustic Sulfidic Spent % — — 9.1 CausticEthylene Spent % — — 73.8 Caustic Analytical Results COD Feed mg/L O₂75,545 38,100 COD Effluent O₂ 17,020 6,125 COD Reduction % — 78.5 83.8pH — — 8.2 8.4

Example 2 Reduction in Caustic pH Vs. COD Removal

Samples of a spent caustic were obtained from a diesel wash process froma refinery and treated in a wet air oxidation unit as shown in FIG. 1.The samples were diluted with water in order to reduce the COD of thesamples and divided into two groups. Control tests of diluted spentcaustic samples all having a pH of greater than 12 were oxidized in aWAO at a temperature of 260° C. and a pressure of about 80 atm for about1 hour. The COD destruction for each ranged from about 60% to about 70%.

The pH of the other group of samples was reduced to about 10.5 by theaddition of sulfuric acid. These samples were oxidized in a WAO at atemperature of 240° C. and pressures ranging from about 60 atm for about1 hour. The COD destruction for each pH adjusted sample was about 80%.The addition of the acid to reduce the pH of the spent caustic generallyimproved efficiency by as much as about 10%, even when oxidized at alower temperature and pressure. This reduction in pH increased CODdestruction efficiency and may also result in lower operating andcapital costs associated with the ability to use less expensivematerials of construction.

Example 3 pH Reduction of Caustic with Sulfuric Acid Vs. with CarbonDioxide

Two samples of refinery spent caustic were treated in a WAO system, eachat 240° C. with a hydraulic residence time of 90 minutes. Prior totreatment, one of the samples was pH adjusted by the addition ofsulfuric acid, while the other was pH adjusted by the addition of carbondioxide. The data presented below in Table 5 was obtained fromcontinuous flow pilot data. It can be seen from this data thatperforming WAO after pH reduction with carbon dioxide resulted in ahigher COD reduction (approximately 80%) relative to the same processperformed with a pH reduction performed by the addition of sulfuric acidto the spent caustic (a COD reduction of approximately 72%). Withoutbeing bound by any particular theory, it is believed that some of theorganics in the spent caustic are converted into acid oils when the pHis lowered using a strong acid such as sulfuric acid. When carbondioxide is used, these organics remain as alkaline salts for a longerperiod of time during the oxidation process.

TABLE 5 pH control pH control with H2SO4 with CO2 Test Number Units 2932 Charge Conditions Reaction Temperature ° C. 240 240 Residence Timeminutes 90 90 Spent Caustic Composition Naphthenic Spent Caustic % 63.063.0 Cresylic Spent Caustic % 9.0 9.0 Sulfidic Spent Caustic % 28.0 28.0Ethylene Spent Caustic % — — Analytical Results COD Reduction % 72.480.4 Organic COD Reduction % 69.4 78.6 pH — 8.1 8.2

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is:
 1. A system for treating a waste stream comprising:a source of a sulfidic spent caustic; a source of refinery spentcaustic; a mixed caustic conduit having an inlet in communication withthe source of sulfidic spent caustic and the source of refinery spentcaustic; and a wet air oxidation unit comprising an inlet and aneffluent outlet, the inlet of the wet air oxidation unit incommunication with an outlet of the mixed caustic conduit; and a heatexchanger arranged to receive an oxidized effluent from the effluentoutlet and a mixed caustic from the mixed caustic conduit, the heatexchanger effective to heat the mixed caustic with heat from theoxidized effluent.
 2. The system of claim 1, further comprising a sourceof a pH adjuster in fluid communication with the wet air oxidation unit.3. The system of claim 2, wherein the source of pH adjuster is a sourceof carbon dioxide.
 4. The system of claim 2, further comprising abiological treatment unit in fluid communication with the effluentoutlet and downstream of the wet air oxidation unit.
 5. The system ofclaim 4, further comprising a source of a pH adjuster in fluidcommunication with the effluent and upstream of the biological treatmentunit.
 6. The system of claim 1, wherein the mixed caustic comprises aCOD concentration of from 20,000 to 125,000 mg/L.
 7. The system of claim6, wherein the refinery spent caustic has a COD concentration of morethan 100,000 mg/L, wherein the sulfidic spent caustic comprises anethylene spent caustic, and wherein the ethylene spent caustic comprisesa COD concentration of 30,000 mg/L or less.
 8. A method for treating awaste stream comprising: combining a sulfidic spent caustic comprisingat least a first organic compound with a refinery spent causticcomprising at least a second organic compound in a ratio sufficient toproduce a combined stream comprising a COD level below that whichresults in the precipitation of carbonate during a wet air oxidation ofthe combined stream and above a level which will allow the wet airoxidation reaction of the combined stream to be self sustaining; andoxidizing by wet air oxidation the combined stream at an elevatedtemperature and a superatmospheric pressure sufficient to treat at leasta portion of the second organic compound.
 9. The method of claim 8,wherein the combined stream COD level is between about 20,000 mg/L and100,000 mg/L.
 10. The method of claim 9, further comprising heating thecombined stream with an effluent from the wet air oxidation unit. 11.The method of claim 10, further comprising treating at least a portionof the effluent in a biological treatment unit.
 12. A system fortreating a waste stream comprising: a wet air oxidation unit comprisingan inlet and an outlet; a source of ethylene spent caustic comprising atleast one oxidizable compound in fluid communication with the inlet ofthe wet air oxidation unit; a source of refinery spent causticcomprising at least one organic compound in fluid communication with theinlet of the wet air oxidation unit; a combined caustic streamcomprising the ethylene spent caustic and the refinery spent causticwithin the wet air oxidation unit, the combined caustic streamcomprising a COD level below that which results in precipitation ofcarbonate upon wet air oxidation of the combined stream; and a heatexchanger arranged to receive an oxidized effluent from the outlet ofthe wet air oxidation unit and the combined caustic stream upstream ofthe wet air oxidation unit, the heat exchanger effective to heat thecombined caustic stream with heat from the oxidized effluent; whereinthe wet air oxidation unit is configured to oxidize the combined streamat an elevated temperature and a super-atmospheric pressure.
 13. Thesystem of claim 12, further comprising a post-treatment unit incommunication with the outlet of the wet air oxidation unit.
 14. Thesystem of claim 13, further comprising: a pH sensor configured to detecta pH of an effluent of the wet air oxidation unit; a controller coupledto the pH sensor; and a source of pH adjuster coupled to the controllerand in fluid communication with the wet air oxidation unit.
 15. Thesystem of claim 14, wherein the source of pH adjuster is selected fromthe group consisting of an acid, a base, and carbon dioxide.
 16. Thesystem of claim 12, wherein the combined caustic stream comprises a CODconcentration of from 20,000 to 125,000 mg/L.
 17. The system of claim12, wherein the refinery spent caustic has a COD concentration of morethan 100,000 mg/L, and wherein the ethylene spent caustic furthercomprises a COD concentration of 30,000 mg/L or less.
 18. A system fortreating a waste stream comprising: a source of a sulfidic spentcaustic; a source of refinery spent caustic; a mixing vessel having oneor more inlets in communication with the sources of ethylene andrefinery spent caustics; a mixed caustic conduit extending from and incommunication with an outlet of the mixing vessel; and a wet airoxidation unit comprising an inlet in communication with an outlet ofthe mixed caustic conduit; and a heat exchanger arranged to receive anoxidized effluent from an outlet of the wet oxidation unit and a mixedcaustic from the mixed caustic conduit, the heat exchanger effective toheat the mixed caustic with heat from the oxidized effluent.
 19. Thesystem of claim 18, wherein the mixed caustic comprises a CODconcentration of from 20,000 to 125,000 mg/L.
 20. The system of 26,wherein the refinery spent caustic has a COD concentration of more than100,000 mg/L, wherein the sulfidic spent caustic comprises an ethylenespent caustic, and wherein the ethylene spent caustic further comprisesa COD concentration of 30,000 mg/L or less.